Difference Between Purines and Pyrimidines

What are Purines and Pyrimidines?

The purines and pyrimidines are the building blocks of DNA and RNA that form heterocyclic, aromatic compounds as well as belong from two distinct nitrogenous bases. They have many similarities with the chemical anatomy of the organic compound pyridine (C5H5N) and are also closely related to benzene (C6H6) since here: a nitrogen atom replaces one Carbon atom. Purines and Pyrimidines also serve as the basis for several other chemical compounds like caffeine, thiamine, theobromine, etc. The Purines consist of compounds like adenine and guanine, while the pyrimidines comprise of popular compounds like cytosine and thymine. 

Pyrimidines primarily have four carbon atoms and two nitrogen atoms, giving it the shape of a ring, as the Nitrogen atoms take the 1st and 3rd place in the ring. The pyrimidines are easily distinguishable like uracil, uric acid, and barbiturates due to such a distinct structure. The pyrimidines are instrumental in the signalling functions of cells, storing energy in the form of phosphates and enzyme regulation, and creating starch and protein. 

Purines, on the other hand, consist of pyrimidines and imidazole rings (also known as a five atom ring with two non-consecutive nitrogen atoms). These have a two-ringed composition with nine atoms overall - five-carbon and four nitrogen atoms. Purines are found in a surplus amount in meat, fishes, and grains, and many other food items like starch and proteins. Being a crucial part of the DNA and RNA structure, they have similar functionalities as Pyrimidines.  


Structure of Purine and Pyrimidine

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The purines and pyrimidines both contain active molecules like the ones present in drugs and vitamins. The purines have a melting point of 214 °C (487K), and the pyrimidines have a melting point of 20-22°C (room temperature). The purine's molar mass is 120.11 g mol-1, and for pyrimidines, the molar mass is 80.088 g mol-1. They are each synthesized in the lab via the Traube Purine Synthesis method and Biginelli Reaction, respectively.

These compounds contain hydrogen bonding between each other and, therefore, link both the strands present in the DNA double helix structure and make parallel structures between DNA and RNA. In the case of DNA, the purine adenine bond formation takes place with the pyrimidine thymine, while the purine guanine forms a bond with the pyrimidine cytosine. For RNA, the adenine bonds with uracil and guanine need to bond with cytosine. Therefore, to establish DNA or RNA, equal proportions of purines and pyrimidines is a pre-requisite. 

Since for DNA and RNA, several other configurations can occur, including that of the methylated pyrimidines, such structures are called 'wobble pairings' as exceptions to the Watson-Crick pairs found in the purine and pyrimidine bases.


Difference Between Purine and Pyrimidine

Purine Catabolism

Purine yields uric acid as the final product in the human body. Simultaneously, other mammals have enzymes like the urate oxidase that form more soluble allantoin as the final product. The guanine nucleotides get hydrolyzed to that of the nucleoside guanosine and are then introduced to phosphorolysis. Since human nucleotidases aren't hyperactive, the AMP is further deaminase to IMP, which is then degraded to yield hypoxanthine. 


Conversion of Base to Uric Acid

The adenine and guanine nucleotides have the common intermediate known as xanthine and form xanthine oxidase. When it occurs in the liver, the guanine is deaminated to release ammonia that is carried as glutamine. The xanthine oxidase is present in large amounts in the liver and intestines. 


Pyrimidine Catabolism

The Pyrimidines are the final products of the catabolism between the beta-amino acids and the ammonia and carbon dioxide. The pyrimidines that are synthesized from the nucleic acids, with the help of nucleotidases and the pyrimidine nucleoside phosphorylase, form the four-amino group of cytosine and five-methylcytosine. It releases ammonia and carbon dioxide.

Since the purines and pyrimidines are heterocyclic, they can come together to form several nitrogenous bases. However, since purines are made up of two rings instead of one, they have a heavier molecular weight than that of others. The circular ring structure plays its role in the melting points and solubility of these compounds. The aforementioned ways represent how these molecules are synthesized and broken down differently by the body in different places, as the purines are manufactured in the liver and the pyrimidines in the tissues. 

Factor

Purines

Pyrimidines

Structure

Double carbon-nitrogen ring with 4 Nitrogen atoms

Single carbon-nitrogen ring with two nitrogen atoms

Size

Larger in size 

Smaller to Purines

Melting Point

214 °C (487K)

20 -22°C (room temperature)

Molar Mass

120.11 g mol-1

80.088 g mol-1

Source

Adenine and Guanine

Cytosine in both DNA and RNA Uracil only in RNA thymine only in DNA

Chemical 

Formula 

C5H4N4

C4H4N2

Solubility in Water

500 g/L  

Miscible

Biosynthesis

It occurs in the liver.

It occurs in various tissues in the human body.

Catabolism Product

Uric acid

Ammonia and Carbon dioxide

Used in

DNA, RNA, vitamins, drugs (e.g., barbiturates), energy storage, controlling enzymes formation

DNA, RNA, drugs (e.g., stimulants), energy storage, synthesis of protein and starch 

FAQ (Frequently Asked Questions)

1. Why are the DNA Purines and Pyrimidines the Same?

According to Szybalski's rules, the number of purine bases in a single strand of DNA is equal to that of the number of complementary pyrimidines. As the coding sequences generate copies as much as 80-90% of the strand, therefore is directly proportional to the length of the coding sequences of the two strands causing the purines and pyrimidines present in equal, complementary amounts. 

2. Where are Purines Found?

The purines are found inside the nucleus of several plants and animal cells. It is known as the building blocks of DNA and RNA as they come from nucleotide acids that are crucial for the strands. They are similar to other organic compounds like Benzene and Pyridine and are found abundantly in meat, fishes, and grains, etc.